Vertical-cavity surface-emitting lasers (VCSELs) and resonant-cavity light-emitting diodes (RCLEDs) are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, and telecommunications. These vertically-emitting devices, with a resonant cavity perpendicular to a surface of a semiconductor wafer on which the devices are fabricated, have many advantages over edge-emitting devices, including the possibility for wafer scale fabrication and testing, and the possibility of forming two-dimensional arrays of the vertically-emitting devices. The circular nature of the light output beams from these devices also makes them ideally suited for coupling to optical fibers as in optical interconnects for integrated circuits and other applications.
VCSELs and RCLEDs have very similar device structures comprising an active region sandwiched between a pair of mirror stacks. A semiconductor p-n or p-i-n junction is formed about the active region; and an electrical injection current is provided across the junction to generate light within the active region. Electrodes above and below the mirror stacks provide an electrical connection to the devices, with one of the electrodes generally defining a central opening for the emission of light in a direction normal to the plane of the active region.
A particular problem in the development of VCSELs and RCLEDs has been the low electrical-to-optical power conversion efficiency heretofore. A low power conversion efficiency is undesirable since it reduces the light output power and produces heat (due to non-radiative recombination processes) within the device structure. Such device heating limits the maximum light output power due to a rollover of the light-vs-current characteristic curve (i.e. a decreasing light output with increasing current), and may even quench the light output completely. In the formation of two-dimensional arrays of semiconductor light-emitting devices, the heat loading from inefficient device operation increases a minimum lateral spacing of the arrayed devices, thereby reducing the manufacturing yield and increasing the array package size and cost.
To increase the power conversion efficiency of VCSELs and RCLEDs, it is necessary to minimize an electrical input power (i.e. an operating voltage and current) required to produce a specified light output power for the devices.
An advantage of the semiconductor light-emitting device and method of the present invention is that the operating voltage of the device may be reduced by periodically varying a semiconductor alloy composition within at least one mirror stack during growth to reduce the series resistance therein, and by reducing any constrictions to current flow in an upper portion of a second mirror stack.
Another advantage of the semiconductor light-emitting device and method of the present invention is that at least one high resistivity or insulating control layer may be provided near the active region to channel the current into a central portion of the active region for more efficient light generation therein.
A further advantage of the semiconductor light-emitting device and method of the present invention is that the control layer may define a lateral refractive index profile for providing a lateral confinement of the light generated in the active region.
Still another advantage of the semiconductor light-emitting device and method of the present invention is that the control layer comprises an annular oxidized portion that extends from an outer edge of the control layer laterally inward towards a central axis of the device more than any other oxidized semiconductor layers in the device, thereby providing less impediment to current flow in the portions of the device above the control layer, especially in the second mirror stack.
Yet another advantage of the semiconductor light-emitting device and method of the present invention is that an inward lateral extent of the annular oxidized portion of the control layer may be defined and increased relative to the inward lateral extent of any other oxidized semiconductor layers that may be present in the device by providing the control layer with an aluminum alloy composition higher than any aluminum composition of the other semiconductor layers.
Another advantage of the semiconductor light-emitting device and method of the present invention is that the annular oxidized portion of the control layer may have an asymmetric shape to provide a preferred direction of polarization for the light generated in the device.
Still another advantage of the semiconductor light-emitting device and method of the present invention is that a first cavity resonance may be provided in an on-axis portion of the resonant optical cavity that has a wavelength the same or different from the wavelength of a second cavity resonance in an off-axis portion of the cavity, thereby providing means for modifying or controlling optical characteristics of the device including one or more polarization states and modes of the light generated within the device.
These and other advantages of the semiconductor light-emitting device and method of the present invention will become evident to those skilled in the art.